At present, wide application of the SAW element is studied and research and development thereof are performed. Attention is paid in particular to an SAW convolver having a real time convolution function e.g. as an important device in a demodulator in the SS communication system, which is expected as a next generation communication system for civil use. In this SS communication system, since the carrier is spread into a wide frequency band by using a pseudo noise (hereinbelow abbreviated to PN) code, it is required for the SAW convolver to have wide band frequency characteristics.
The basic structure of the SAW convolver stated above consists of two interdigital electrodes 2 and 2' formed on a piezoelectric substrate 1 and an output gate electrode 3 formed therebetween as indicated in FIG. 7. The convolution operation is effected by transforming two input signals f(t) and g(t) into SAWs by means of the input interdigital electrodes 2 and 2', respectively, which SAWs propagate in an interaction region under the output gate electrode in directions opposite to each other to be multiplied by each other by using an elastic non-linearity or the non-linear property of semiconductor and integrated by the gate electrode.
The SAW convolver described above is classified into 3 sorts, depending on the structure thereof, i.e. they are known as an elastic convolver utilizing an elastic non-linear property, an air gap type convolver utilizing the non-linear property of semiconductor, in which a piezoelectric body and a semiconductor are brought close through an extremely small gap, and a monolithic type convolver, in which a piezoelectric thin film is superposed on a semiconductor substrate to form a monolithic body. It is thought that the most advantageous structure among them is the monolithic type utilizing the non-linear property of semiconductor, with which a high-efficiency can be obtained and no extremely small gap is required.
However, since the piezoelectric substrate of multi-layered structure constituted by a plurality of materials, in which the sound velocity differs, has a velocity dispersion, by which the sound velocity of the SAW differs, depending on the frequency, the monolithic type convolver described above has a problem that the working frequency band is restricted and that it is difficult to widen the frequency band. Further the design taking the compensation of this velocity dispersion into account is extremely complicated.
Further in an SAW convolver having a prior art structure, since the output gate electrode effecting the convolution operation is either a single rectangular electrode or a plurality of strip-shaped electrodes, which are parallel to the propagation direction of the SAW and have a same length, the effective integration time differs, depending on the frequency in a wide frequency band, which is a cause of worsening spurious characteristics.
Contrarily thereto, another structure is proposed, in which chirp type electrodes 4 and 4', where the pitch of the fingers constituting the interdigital electrodes is varied continuously along the propagation direction of the SAW so that the position where the SAW is excited is varied, corresponding to the frequency, as indicated in FIG. 8, are used for the input transducers in order to compensate the velocity dispersion. However, since all the components in the frequency band use essentially a same propagation path in common, the phenomenon that the integration time differs, depending on the frequency, cannot be avoided.